Prior art toasters have conventionally employed a bi-metallic element to time the toast cycle, the bi-metallic element bending as it is heated during a cycle and eventually closing a pair of contacts to energize a solenoid which in turn releases a spring loaded lift mechanism so that the toast is raised out of a bread well and a switch is opened to terminate power. Others have proposed the use of solid state heat sensors for controlling the toast cycle. With such arrangements it is difficult to control the toaster such that each cycle toasts to the desired preselected color.
As exemplified by Iida et al. U.S. Pat. No. 3,431,400, Rolland U.S. Pat. No. 4,518,849 and Schneider U.S. Pat. No. 4,510,376, some toasters avoid the use of the bi-metallic element by using an analog electrical timing circuit for controlling the toast cycle time. In addition, Schneider U.S. Pat. No. 4,510,376 purports to compensate to a lesser degree for the differences in the preceding heating cycle time and cool down interval. However, the known prior toasters do not fully take into account the preceding cycle time, the mode during the previous cycle, the number of previous toasting cycles occurring within a few minutes of the start of a given cycle, the color to which bread was toasted during the preceding cycle, or whether or not the last cycle was manually interrupted before completion. Furthermore, those which do not rely on temperature sensors are not readily adaptable for use as multi-mode toasters.
Norwood U.S. Pat. No. 4,345,145 discloses a toaster oven employing a combination of digital and analog circuit techniques. In this device an integrator is utilized to control cycle time such that a desired toast color is obtained independently of the temperature of the toast compartment at the beginning of the cycle.